Subduction zones are a type of convergent margin where one portion of the crust is driven beneath another. This phenomenon can occur at continent-continent convergent margins (where continental crust is subducted beneath continental crust, creating the mountains that we observe on the surface of the Earth today), oceanic-oceanic convergent margins, and oceanic-continental convergent margins, which are what I currently study. At the surface, these oceanic-continental subduction events are the catalyst for many crucial geologic processes, including volcanism (which creates new continental crust), and earthquakes.
The figure depicted above is the subduction of the Farallon Plate beneath the North American Plate during a Cretaceous (145.5-65.5 million years ago) subduction event where California (USA) is today. The area of this subduction zone that I'm interested in is the highlighted region between the Farallon and North American Plates: the subduction zone interface (also known as the subduction channel by some). This interface acts as the "catalyst zone" for many of the surface (and sub-surface) processes I mentioned in the previous paragraph. As hydrated crustal material moves along the interface and interacts with the dry lithosphere, a series of important chemical reactions occur. This process adds water to the mantle, which lowers the melting temperature of the mantle minerals, causing magma to form. While the primary event that results from this mantle hydration is the volcanism that forms new continental crust, it has also been shown to cause seismic activity (earthquakes).
Exhumed (or 'brought to the surface') rocks from the Farallon-North American subduction interface (and others around the World), have led Geologist to ask questions about how material behaves at the interface, including "does the material flow, or does it behave as a coherent convecting unit?" To answer these questions , we turn the Catalina Schist, located on Santa Catalina Island (California, USA), which has been interpreted as an exhumed portion of the Farallon-North American subduction interface. For the most part, the exposure preserves distinct zones of rocks that experienced the same pressure and temperature conditions during metamorphism (refered to here-on-out as facies), which suggests that the material moved fairly coherently. There are, however, some "exotic block" exceptions, where higher-grade rocks are preserved within a lower-grade matrix or in contact with lower-grade rocks. To-date, few studies have attempted to explain how these exotic blocks found themselves in contact with a lower-grade matrix. John Platt, in his 1975 and 1976 papers, suggests that the blocks moved from the amphibolite-facies zone to their current position along post-metamorphic thrust faults during exhumation of the interface. His observations were based on his observations during his mapping of Santa Catalina Island, and to-date we have found several new occurrences of the exotic blocks. While many that John described occur along fault contacts or in shear zones (which you would expect based on his hypothesis), many of the new occurances that we have mapped are not. This suggests that other mechanisms may have played a role in their emplacement, and that's where I and my geochemical toolbelt enter!
We can use a variety of techniques, including trace element thermometry, raman spectroscopy of mineral inclusions, and U-Pb geochronology to reconstruct the pressure-temperature-time (P-T-t) history of these exotic blocks, the surrounding low-grade material, and the amphibolite-facies zone rocks. This will aid us in understanding how these blocks behaved within the subduction zone, enabling us to test new hypotheses about subduction zone interface dynamics!
Work supported by NSF Grant EAR-1419871 (PIs S. Penniston-Dorland, M. Kohn)
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